A telephone and a facsimile machine have a function called Caller ID service (which is also called “number display service”) for displaying caller (originator) information when receiving an incoming call. The caller information may be a telephone number of the caller or a name of the caller. This function enables the recipient to know the caller before taking the call when a call comes in. Accordingly, the recipient can be prepared to answer the phone or to reject reception depending on the caller. If the recipient cannot answer the phone or a message is not left on the recipient's answering machine, the caller information is recorded and the recipient can call back the caller afterward by performing simple operation.
The Caller ID function is performed according to a communication procedure shown in FIG. 6. As shown in FIG. 6, if a caller originates a call, the polarity of the current flowing on the telephone line is inverted in a local exchange (simply, exchange), and after the expiration of a predetermined time of 0.1 seconds or more, an information reception terminal activation signal (simply a terminal activation signal, or CAR signal) is transmitted from the exchange to an information reception terminal (simply, terminal). The CAR signal is a 16-Hz signal transmitted intermittently every 0.5 seconds with a 0.5-second interval.
Upon reception of the CAR signal, the called terminal connects the telephone line to form a DC loop with the exchange and switches from communications based on V.21 standard to communications based on ITU-T Recommendation V.23 standard for receiving an FSK (Frequency Shift Keying) signal.
The exchange detects formation of the DC loop as a primary response signal and if the exchange receives a primary response signal from the terminal within 6 seconds after inversion of the polarity, the exchange transmits caller information by a signal conforming to the ITU-T Recommendation V.23 standard.
Upon completion of reception of the caller information from the exchange, the called terminal restores the communications to communications based on the V.21 standard and displays the caller information, such as the telephone number of the caller, on a display. At the same time, the called terminal disconnects the telephone line to disconnect the DC loop. If the exchange detects disconnection of the DC loop as a reception completion signal of the telephone number data, the exchange sends a general ringing signal.
The ringing signal is a 16-Hz signal transmitted intermittently every second with a 2-second interval. While transmitting the ringing signal, the exchange transmits a ring back tone (RBT) to the caller (originator).
If the called terminal is a telephone, the telephone rings in accordance with the ringing signal from the exchange and if the user lifts a handset of the telephone off the hook, the telephone line is connected back and a DC loop with the exchange is formed. If the exchange detects the DC loop as a secondary response signal, the exchange restores the inverted polarity of the telephone to a normal polarity to enable the communication.
There is also known a function called “call waiting” for displaying caller information of the caller when a new call comes in during a communication with another call. This function enables a user to know the caller information of the caller of the new call received during the communication with another call and enables the recipient to determine whether an urgent response to the new call is required.
The call waiting display service is performed according to a communication procedure shown in FIG. 7. As shown in FIG. 7, when an interrupter of a third party originates a call while a user is in communication with another communication party, the exchange sends an origination (calling) display tone to the terminal and also transmits a ring back tone (RBT) to the interrupter. The exchange transmits a DTMF (Dual Tone Multi Frequency) signal of an information reception terminal activation tone (simply, a terminal activation tone) in about 1 second after transmission of the origination (calling) display tone. The terminal activation tone is formed by a tone indicating “D” following “C.” The exchange transmits caller information by an FSK (frequency modulation) signal conforming to the ITU-T Recommendation V.23 following the terminal activation tone.
Upon detection of the terminal activation tone, the terminal switches from communications based on the V.21 standard to communications based on the ITU-T Recommendation V.23 standard for receiving an FSK signal, receives the telephone number of the interrupter, and displays the telephone number on the display. Upon completion of reception of the telephone number, the terminal restores the communication to the communication based on the V.21 standard. While communication is set to the V.23 standard, the communication line becomes silent.
FIG. 8 shows the format of data transmitted by an FSK signal in the Caller ID or call waiting display service. As shown in FIG. 8, the data is formed at the top with consecutive mark bits of 1. The time period for the transmission of the mark bits is set to be 60 msec or more. Formed following the mark bits are a transparent mode control signal DLE added preceding a control signal, a control symbol (SOH) indicating the beginning of a header, a header indicating information sending from exchange to terminal, DLE, a control symbol STX indicating the start of text data, text data indicating the telephone number or caller name for conducting Caller ID, DLE, a control signal ETX indicating the end of the text data, and CRC (Cyclic Redundancy Check) for error correction. The text data includes information indicating service type, parameters, and parameter types.
A document JP-A-10-224587 discloses an apparatus for receiving a caller (originator) number notification and storing the caller (originator) number in a memory for displaying the caller (originator) number on a display.